1. Field of the Invention
The present invention relates generally to spin valve sensors, and more particularly to a spin valve sensor having a pinned layer which is longitudinally biased by antiferromagnetic (AFM) pinning layers in end regions outside the central active sensor region.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks are commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR heads, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an “MR element”) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which the MR element resistance varies as the square of the cosine of the angle between the magnetization of the MR element and the direction of sense current flow through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Within the general category of MR sensors is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers. GMR sensors using only two layers of ferromagnetic material (e.g., nickel-iron, cobalt-iron, or nickel-iron-cobalt) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
In an SV sensor, one of the ferromagnetic layers, referred to as the pinned layer, has its magnetization typically pinned by exchange coupling with an antiferromagnetic (AFM) layer (e.g., nickel-oxide, iron-manganese, or platinum-manganese) layer. The pinning field generated by the AFM layer should be greater than demagnetizing fields to ensure that the magnetization direction of the pinned layer remains fixed during application of external fields (e.g. fields from bits recorded on the disk). The magnetization of the other ferromagnetic layer, referred to as the free layer, however, is not fixed and is free to rotate in response to the field from the information recorded on the magnetic medium (the signal field). The pinned layer may be part of an antiparallel (AP) pinned layer structure which includes an antiparallel coupling (APC) layer formed between first and second AP pinned layers. The first AP pinned layer, for example, may be the layer that is exchange coupled to and pinned by the AFM pinning layer. By strong antiparallel coupling between the first and second AP pinned layers, the magnetic moment of the second AP pinned layer is made antiparallel to the magnetic moment of the first AP pinned layer.
In a self-pinned spin valve sensor, however, the first AP pinned layer is not pinned by the AFM layer but is rather “self-pinned”. A spin valve sensor of this type relies on magnetostriction of the AP self-pinned layer structure and the air bearing surface (ABS) stress for a self-pinning effect. An AFM pinning layer, which is typically as thick as 150 Angstroms, is no longer necessary for pinning so that a relatively thin sensor can be advantageously fabricated.
Utilizing a self-pinned structure in a spin valve sensor achieves higher bit densities with its thinner profile and increased sensitivity. One of the problems for self-pinned spin valves, however, has been to improve the pinning field against “flipping”. Readback signals from the disk are detected as either a “0” or “1” depending on the polarity of the bits recorded on the disk. However, when an undesirable head-to-disk interaction occurs (due to defects, asperities, bumps, etc.), the sensor experiences compressive or tensile stress which causes the pinned layers to flip orientation. Electrical overstress (EOS) from electrostatic discharge (ESD) in the sensor during manufacturing and/or handling may also induce such flipping. The pinned layers may flip their direction either permanently or semi-permanently depending on the severity of the stress. This causes the amplitude of the readback signal to flip (hence the terminology “amplitude flip”), which results in corrupt data. While the self-pinned structural approach decreases the occurrence and severity of amplitude flipping amongst self-pinned spin valve heads, an unacceptably large fraction still exhibit amplitude flipping into the negative polarity state.
Accordingly, there is an existing need to overcome these and other deficiencies of the prior art.